I would suggest the filter from Ray Wilson's Weird Sound Generator. It is really simple, has a low parts count, is single supply (you can run it off of the same battery as your APC) and uses very common parts. For a simple filter it works great. The schematic is at musicfromouterspace.com.
If you want something with a little more "punch" and don't mind a bipolar supply, a Sallen-Key filter like Squarewaveparade's sounds pretty good to my ear. His schematic is around here somewhere, but you can also google "sallen-key" and come up with workable schematics...
Do a search on the forum here, there are at least three or four filters which I think can be breadboarded or put together by a relative beginner.
Good luck, hope this helps.

O.k., so the filter section (labeled ODD) is everthing from C1 (the 0.1 uf capacitor) to the right. It is built around the 741 op-amp (labelled U2 LM741 in the schem). If you put together everything from C1 through the end of the circuit (output) you should be good. Just run your output from the apc (or other source) into C1.
(full disclosure: I have built the filter from the original wsg, not the current revision, but I am assuming it should still work.)
As for the Sallen-Key, let me check what I've got on my breadboard, I may be able to post a simple schematic in the next couple of days. In the meantime, you might try Nicolas's state-variable lowpass from the links at the top of this page. It looks pretty simple and has a perfboard layout as well as schematic...
Good luck, hope this helps.[/url]

well, I finished my schematic with appropriate component values. I'm attempting to attach it in this post. (I'm not so net savvy, so I type this, post it, see what happens, and if I flub it I'll edit and try again...)
The funny looking pot on the left is a dual (stereo) 100k pot (which will control cutoff). The points are labelled and should be connected to the corresponding points in the circuit.
The resonance pot shown is 100k. It will start oscillating at about 50%, this can be very unpleasant. You can use a 50k here and it will cut before self-oscillating. I like the 100k better because you can get squelchier highs out of it.
Please note this is not set up for voltage control, you have to physically play with the two pots to get sweeps.
I take no credit for the design of this circuit, it is culled from a couple of public sources, however, if there are errors they are my own.
Please note that the points labelled "C" and "Y" on the dual pot must be connected.

Anyway, have fun with this one, to me it sounds great, especially for how simple it is.
good luck.

so... I liked this so much on the breadboard that I went ahead and soldered it down. Here is a short audio sample so you get some idea of the results.

The source is a Boss Dr-5 from circa 1995. The only effect on this recording is the filter.
For your application I can't see not making this. It is small, cheap, and a really simple build. (Breadboard it in less than an hour, build it complete in one go...)

Yes, the schematic is "funny." I decided to draw it up like that because when I first built this circuit the plans I had were very confusing to me (I am a real beginner at this stuff.) Knowing how the dual pot needs to be connected (to itself and the circuit) initially gave me a lot of problems. The schematics I had also did not list component values, so it took some trial and error to get a "good" sound. I hope my drawing isn't too confusing, especially since I did it that way to be easier for us beginners...

If you want to share this schematic please just provide a link to this forum topic.

heheh, sorry for the confusion. I mentioned the wiring of the dual gave me trouble...
1) yes, you need a dual pot, not just double the value. The Sallen-Key topology depends on two equal resistors, the dual pot in this case serves that purpose.
2) yes, the schematic is wrong, sorry for that. ERASE THE LINE CONNECTING "B" and "Y". Also ERASE THE LINE CONNECTING "Y" and "Z".

remeber, though, Y connects to two places, it is jumpered to point C of the dual, and it also is connected to Capacitor 1.

I'll try to get a corrected schematic drawn to replace the faulty one. Sorry for the inconvenience, and thank you for giving this circuit a go.
c_z

I'm glad your getting sound out of it. Try changing your capacitors to 0.01uf ceramic disc capacitors. They should be labelled "103." Hopefully that will clean up the sound a bit.
The volume of your input signal can also affect the amount of distortion.
Keep us posted.
Good luck, I hope this helps.

WOW, that redrawn schematic looks great!
Have you gotten a better sound or is it still distorting heavily? Some sound samples once it's finished are always welcome.
I'm a real beginner, I've never done any PCB's, just breadboard/ verobard type stuff, maybe someone here with more experience can give you some good feedback. Let us know how it goes.
cheers,
c_z

Not bad for your first attempt at laying out a PCB. If you don't mind, I'll offer a couple of hints and suggestions:

1) Label the positive Output terminal as "Out" and the ground as GND. The plus symbol that's there now is a little confusing, especially since it's close to a power input pad. Same thing with the Input terminal - label it as "In" rather than "+" and label the Ground terminal as GND.

2) Play with the layout a bit and see if you can't get the Y, B and Z terminals all next to each other. Same thing with the P1a, P1b and P1c terminals for the Resonance pot. This will make the wiring cleaner and easier. One routing trick you can try is to run a trace under a resistor or capacitor, esentially using them as 'jumpers.'

3) Make your traces and pads fatter. Easier to solder and easier to etch if you do your own boards.

4) Definitely make the power traces fatter. Reroute them and put them together if possible. Generally, you should route the power traces first (even though they're really short on this board). I try to get all of my connectors to the edges of the board, and keep connectors grouped together where it makes sense.

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